Transcript Atmospheric Chemistry Division National Center for Atmospheric

Air Pollution and Atmospheric Chemistry
Sasha Madronich
Atmospheric Chemistry Division
National Center for Atmospheric Research
Boulder, Colorado USA
27 July 2004, Boulder
1
Components of Air Quality Models
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Spatial and Temporal Grids
• Horizontal domain (local; regional; global)
• Vertical extent (PBL; troposphere; trop+strat+mesosphere)
• Time span (day or week episode; interannual; climatologic)
Chemical Inputs
• Natural emissions
• Anthropogenic emissions
• Inflow from model boundaries
• Initial conditions
Chemical Transformations
• Gas phase
• Condensed phase (aerosols, clouds)
Transport
• Horizontal advection
• Vertical diffusion and convection
• Update environment (T, P, H2O, hn)
Deposition
• Wet (rain, snow)
• Dry (gas & aerosol on surfaces)
Solution forward in time
• Coupled non-linear stiff differential equations
2
Earth’s Atmosphere
 Composition
• 78% nitrogen
• 21% oxygen
• 1-2% water (gas, liquid, ice)
• trace amounts (<< 1%) of many other species, some natural
and some “pollutants”
 Reactivity dominated by
• oxygen chemistry
• solar photons
 To understand fate of pollutants, must first understand oxygen
photochemistry
3
Energetics of Oxygen in the Atmosphere
DHf (298K)
kcal mol-1
Excited atoms
O*(1D)
104.9
Ground state atoms
O (3P)
59.6
Ozone
O3
34.1
“Normal” molecules
O2
0
Increasing
stability
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Atmospheric Oxygen
Thermodynamic vs. Actual
1
1E-10
Concentration, atm.
1E-20
1E-30
O2 (=0.21)
1E-40
thermodyn. O3
thermodyn. O
1E-50
thermodyn. O*
1E-60
1E-70
observed O3
inferred O
1E-80
inferred O*
1E-90
1E-100
1E-110
200
220
240
260
280
300
Temperature, K
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Photochemistry
 Thermodynamics alone cannot explain
atmospheric amounts of O3, O, O*
 Need
– energy input, e.g.
O2 + hn  O + O
(l < 250 nm)
– chemical reactions, e.g.
O + O2 (+ M)  O3 (+ M)
= Photochemistry
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WMO, 2002
Stratospheric Odd Oxygen (Ox = O + O3)
Chapman, 1930’s: Pure oxygen photochemistry
 O3 production:
O2 + hn (l < 240 nm)  2 O
O + O 2 + M  O3 + M
 O3 destruction:
O3 + hn (l < 800 nm)  O + O2
O + O3  2 O2
Correctly predicts vertical profile shape, but too much O3.
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Stratospheric Odd Hydrogen (HOx = OH + HO2)
Bates and Nicolet, 1950’s: Hydrogen-containing “contaminants”
 Formation of excited oxygen atoms:
O3 + hn (l<330 nm)  O2 + O*
 Formation of HOx radicals from H2O and CH4:
H2O + O*  OH + OH
CH4 + O*  OH + CH3
 Catalytic destruction of O3 by HOx:
O3 + OH  O2 + HO2
O + HO2  O2 + OH
O3 + HO2  2 O2 + OH
Better, but still too much O3
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Stratospheric Odd Nitrogen (NOx = NO + NO2)
Crutzen, 1970: Nitrogen containing “contaminants”
 Formation of excited oxygen atoms:
O3 + hn (l<330 nm)  O2 + O*
 Formation of NOx radicals from N2O:
N2O + O*  NO + NO
 Catalytic destruction of O3 by NOx:
O3 + NO  O2 + NO2
O + NO2  O2 + NO
works for natural stratosphere
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Stratospheric Halogens (Cl, Br, I, …)
Rowland and Molina, 1974: Chlorofluorocarbons (CFCs) can make it to
stratosphere because they are not destroyed in troposphere:
 Formation of chlorine atoms from photolysis of
chlorofluorocarbons:
CH3Cl + hn  CH3 + Cl
CF2Cl2 + hn  CF2Cl + Cl
 Catalytic destruction of O3 by Clx:
O3 + Cl  O2 + ClO
O + ClO  O2 + Cl
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Stratospheric Reservoirs
 Formation of less-reactive reservoirs:
Cl + CH4  HCl + CH3
ClO + NO2 + M  ClONO2 + M
OH + NO2 + M  HNO3 + M
 Reservoirs can either be removed by diffusion to troposphere, or
can be transformed back to reactive species.
 Strong reactivation of halogens occurs on surfaces of polar
stratospheric clouds.
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SOLAR SPECTRUM
UNEP, 2002
Detrimental Effects of UV Radiation
 Human and animal health
– Skin cancer, skin ageing, sunburns
– Ocular damage
– Immune system suppression
 Reduced Growth in Plants
– Terrestrial (agriculture, forests)
– Marine (less phytoplankton)
 Air Quality
– More UV means more urban ozone, secondary aerosols
 Materials
– Degradation of plastics (PVC, PC)
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Global UV Changes (1990’s/1980’s)
Clear sky
(ozone change only)
All conditions
(ozone and cloud changes)
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Atmospheric Halogens are Decreasing or Stabilizing
WMO, 2002
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The
Future
Avoided
WMO, 2002
WMO, 2002
Tropospheric Ozone Formation – how?
 Urban ozone (O3) is generated when air containing
hydrocarbons and nitrogen oxides (NOx = NO + NO2)
is exposed to UV radiation (Haagen-Smit, 1950’s).
 Laboratory studies show that O3 is made almost
exclusively by the reaction:
O2 + O + M  O3 + M
 But troposphere lacks short-wavelength photons
(l<250 nm) needed to break O2 directly.
So: what is the source of tropospheric O atoms??
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Tropospheric O3 - From NO2?
 NO2 photolysis is a source of O atoms:
NO2 + hn (l < 420 nm)  NO + O
O + O2 + M  O3 + M
 Two problems:
Reversal by NO + O3  NO2 + O2
Usually O3 >> NO2
 Makes some O3, but not enough!
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Tropospheric O3 Formation – Need hn, HCs, NOx
 Initiation by UV radiation (Levy, 1970):
O3 + hn (l < 330 nm)  O*(1D) + O2
O*(1D) + H2O  OH + OH
 Hydrocarbon consumption (oxygen entry point):
OH + RH  R + H2O
R + O2 + M  ROO + M
 Single-bonded oxygen transferred to NOx:
ROO + NO  RO + NO2
 NOx gives up oxygen atoms (as before):
NO2 + hn (l < 420 nm)  NO + O
O + O2 + M  O 3 + M
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Tropospheric O3 Formation – Secondary Reactions
 Propagation
RO + O2  R’CO + HO2
HO2 + NO  OH + NO2
more O3, OH
 Termination
OH + NO2 + M  HNO3 + M
HO2 + HO2 + M  H2O2 + M
HO2 + O3  OH + 2 O2
slows the chemistry
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Tropospheric Chemical Mechanisms
 This talk:
15 reactions
 Typical 3D model used for air quality:
100 - 200 reactions
 Typical 0D (box) models used for sensitivity studies:
5,000 - 10,000 reactions
 Fully explicit (computer-generated) mechanisms:
106 - 107 reactions
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Hydrocarbon Chemistry is Complex!
1.E+07
Reactions
1.E+06
n-alkanes
1.E+05
i-alkanes
1.E+04
1-alkenes
Species
1.E+03
isoprene
1.E+02
2
3
4
5
6
7
8
9
Number of carbons
Aumont and Madronich, 2003
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Consequences of tropospheric O3 chemistry - 1
 Surface O3 pollution
Urban: 100-500 ppb
Regional: 50-100 ppb
Global background increase
10-20 ppb  35-45 ppb in NH
10-20 ppb  25-35 ppb in SH
 Damage to health and vegetation
 Greenhouse role of O3
 Changes in global oxidation capacity
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California EPA, 2004
Consequences of tropospheric O3 chemistry - 2
 Formation of peroxides and acids:
HO2 + HO2  H2O2 + O2
OH + NO2 + M  HNO3 + M
OH + SO2  …  H2SO4
H2O2(aq) + SO2(aq)  …  H2SO4(aq)
 Damage to vegetation and structures (acid
precipitation)
 Sulfate aerosol formation (visibility, climate)
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Consequences of tropospheric O3 chemistry - 3
 Products of hydrocarbon oxidation
CO2 (minor compared to direct emissions)
CO (~ 1/2 of total global emissions)
Oxygenated organics: aldehydes, ketones,
alcohols, organic acids, nitrates, peroxides
 Damage to health, vegetation
 Secondary organic aerosol formation (health,
visibility, climate)
 Changes in global oxidation capacity
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Global Oxidation (self-cleaning) Capacity
Solar UV radiation
Oxidation, e.g.:
CH4 + OH  … CO2 + H2O
Insoluble  Soluble
Halocarbons
CH4 CmHn
NO NO2
Carboxylic acids
H2SO4, SO4=
Emissions
Deposition
(dry, wet)
HNO3, NO3HCl, Cl-
CO SO2
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Consequences of tropospheric O3 chemistry - 4
 OH increase because of increasing emissions of
NOx?
 OH increase because of increasing UV radiation?
OR
 OH decrease because of increasing emissions of
CO, CmHn, SO2, and other reduced compounds?
 Decreased OH (oxidizing capacity) implies generally
higher amounts of most pollutants including:
• Higher amounts of greenhouse gases
• Higher amounts of substances that deplete the
ozone layer
• More global spread
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How Climate Change Can Affect Pollution - 1
 Changes in Anthropogenic and Biogenic
Emissions:
• Anthropogenic emissions of ozone precursor compounds (CO,
NOx, SOx, NMHC) and aerosols are expected to increase over
the next 50 years.
• Biogenic emissions of NMHCs and CO are expected to be
affected significantly by future changes in temperature, relative
humidity and photosynthetically available radiation (PAR).
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How Climate Change Can Affect Pollution - 2
 Changes in Transport:
•
•
•
•
Modification of inter-continental transport of
pollutants.
Modification of moist convective activity, with
associated changes in wet removal processes and
vertical redistribution of pollutants.
Modification of the boundary-layer height and
ventilation rates.
Modification of stratosphere-troposphere exchange,
with consequently different inputs of ozone to the
troposphere.
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How Climate Change Can Affect Pollution - 3

Changes in Chemically Relevant Environmental Variables:
•
Increased temperatures lead to faster kinetics of O3 production.
•
Changes in H2O, affecting both the gas phase chemistry, e.g. OH
production via O(1D) + H2O, and the growth of aerosols near the
deliquescence point.
•
Changes in cloud distributions, with associated changes in aqueous
chemical processes (e.g. sulfate formation), NOx production by
lightning, wet removal, and photochemistry.
•
Increased aerosol loading, with associated enhancements of
heterogeneous chemistry, and – depending on aerosol type – either
increased or decreased photochemistry.
•
Changes in stratospheric ozone, with associated changes in
photochemistry.
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INTERACTIONS:
Climate change
&
Stratospheric
ozone
WMO, 2002
INTERACTIONS: Climate, Clouds, and UVR:
2130 – Present, SH Summer
Madronich, Tie, Rasch, unpubl.
INTERACTIONS: Climate & Air Pollutants
IPCC, 2001
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IPCC, 2001
INTERACTIONS: Heat, Air Pollution & Health
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INTERACTIONS: Carbon cycle & Tropospheric O3
Loya et al., Nature, 425, 705, 2003
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(a very incomplete picture)
Good?
Bad?
Unclear?
Stratospheric Ozone
Depletion
+ halocarbons
+ H2O
-T
± H2O
+ OH
+ IR cooling
+ CFC replacement
+ UV
+CFC replacement
+ CH4, + O3, + soot,
+ sulfate, ± clouds
Air Quality
Climate Change
+ T, + H2O, ± emissions,
± rain, ± winds, ± clouds